While several anti-retroviral compounds are already known, very few acyclic nucleosides (derivatives lacking an oxacyclopentane) have been reported to exert significant inhibition against human immunodeficiency virus type 1 (HIV-I) in vitro. We found two unsaturated acyclic nucleoside derivatives, adenallene (N9-(4'- hydroxy-1',2'-butadienyl)adenine) and cytallene (N1-(4'-hydroxy- 1',2'butadienyl)cytosine), which protect various CD4+ T-cell lines from the infectivity and cytopathic effect of HIV-I. These compounds inhibit the expression of HIV-I gag protein and suppress viral DNA synthesis at concentrations that do not affect functions of normal T cells in vitro. They also inhibit the in vitro infectivity of another human retrovirus, HIV-2. Further in vitro analyses of the anti-HIV-I activity revealed that the presence of two cumulated double bonds between the 1' and 2' carbons and between the 2' and 3' carbons confers anti-retroviral activity in certain pyrimidine or purine derivatives containing a four carbon chain. We have also found that the 4'hydroxyl group is critical for the in vitro anti-HIV activity of adenallene. Our observations may provide new structure/activity relationships for acyclic nucleoside analogues, and may be of value in developing a new class of experimental drugs for the therapy of HIV-related diseases. The structure/activity relationships discovered so far are unexpected.